Manufacture of silver halide emulsions



United States Patent 3,153,593 MANUFACTURE OF SILVER HALIDE EMULSEGNS Alfons Jozef De Pauw, Edegem, Belgium, assignor to Gevaert Photo-lroducten N.V., Mortsel, Belgium, a Belgian company No Drawing. Filed May 7, 1962, Ser. No. 192,938 Claims priority, application Belgium, May 8, 1961, 40,625, Patent 603,491 4 Claims. (Cl. 96-94) This invention relates to a process for the manufacture of light-sensitive silver halide emulsions.

According to the most usual method of preparing gelatino silver halide emulsions it is known to form lightsensitive silver salts by reacting an aqueous solution of a halide, e.g., potassium bromide with an aqeuous solution of a silver salt, e.g., silver nitrate in the presence of a protective colloid, e.g., gelatin.

The formed silver halide dispersion is ripened which means that the dispersion is kept for a certain time at a certain temperature in order to obtain the desired grain size and grain distribution. In order to wash the silver halide dispersion, the dispersion is first jellified by cooling. The formed gel is noodled or finely divided in another way and the salts present are extracted from the noodles with water at low temperature by dialysis. After removing the salts and the supernatant Water the washed emulsion is melted by heating whereupon gelatin and other ingredients such as chemical sensitizers are added. Thereupon the emulsion is chemically ripened. Then optical sensitizers, color couplers, hardeners, anti-fogging agents, wetting agents, matting agents, etc. can be added.

This process, however, has the great disadvantage of needing considerable time for setting, noodling and washing. Moreover, this process has to be carried out in the presence of a relative large amount of binding agent which cannot be decreased at will.

For setting and noodling a concentration of at least 4% of binding agent is necessary. In some cases, e.g., in the preparation of X-ray emulsions a relatively large amount of light-sensitive silver halide per unit of area is necessary. Such emulsions can be concentrated by evaporating the water present whereby, however, the ratio of binding agent to light-sensitive salt remains unchanged. Hence a. more concentrated emulsion is only obtained together with an increased viscosity which hinders the coating of the emulsion so that, rather than concentrating the emulsion too much, it is usually preferable to apply several emulsion layers one upon another in order to obtain the desired amount of silver salt per unit of area.

It is also known that the gelation and the noodling of the above mentioned photographic emulsion can be avoided by coagulating this emulsion either by adding a large amount of a water miscible organic solvent wherein the binding agent does not dissolve or by salting out the binding agent by the adding of salts such as lead, titanium, thorium, zirconium, ammonium, barium, calcium, sodium and zinc salts. By these methods, the binding agent present precipitates together with the lightsensitive silver salt. This precipitate still contains a part of the undesired by-products and of the excess of halogen salts.

These undesired salts can be removed from the precipitate by washing with water; this is, however, diflicult to carry out and time-consuming since the precipitate is gummous and sticky because of the large amount of gelatin so that it is practically impossible to Wash out this emulsion completely without mechanical dividing.

Another process for salting out is known from the American patent specificaiton 2,618,556 wherein is described how a gelatino silver halide dispersion is formed 3,153,593 Patented Get. 20, 1964 by mixing a water-soluble silver salt and a water-soluble halogen salt in an aqueous gelatin solution. After ripening the emulsion is precipitated in a washable form by the adding of a water-soluble anorganic ammonium salt and a sufficient amount of a water-soluble acid with an ionization constant at least equal to that of acetic acid in order to decrease the pH at least to 6.

It has now been stated that by the precipitation of the light-sensitive silver halide in a medium with pH lower than 7 in many cases, especially on preparing high sensitive ammoniacal emulsions, the sensitivity is considerably decreased.

It has now been found that it is possible to obtain an easily washable gelatin silver halide coprecipitate even when carrying out the flocculation at a pH not below 7, if

(1) At the end of the formation of the silver halide the gelatin content of the dispersion is not higher than 2% and the weight ratio of gelatin to silver nitrate (calculated on an equivalent weight basis to the silver halide) varies between A and 7 and (2) The gelatin silver halide coprecipitate is hardened before the washing step by treating it with a concentrated aqueous solution of ammonium sulfate.

The emulsions prepared in such circumstances are not decreased in sensitivity.

More particularly it is an object of this invention to provide a process for the manufacture of high sensitive neutral and ammoniacal silver halide emulsions comprising:

(1) Forming and dispersing silver halide grains in a neutral aqueous respectively ammoniacal medium, wherein at the end of the formation of the silver halide the gelatin content of the dispersion is not higher than 2% and the ratio of gelatin to silver nitrate varies between /100 and /100,

(2) Physically ripening the silver halide,

(3) Coprecipitating the gelatin and the physically ripened silver halide, by adding a concentrated solution of ammonium sulfate, whereby the pH of the medium is not decreased below pH 7,

(4) Draining off the supernatant liquid,

(5) Reversibly hardening the gelatin which has been ooprecipitated with the silver halide by adding and a concentrated aqueous solution of ammonium sulfate and stirring therewith,

(6) Removing the soluble salts by washing and decanting,

(7) Redispersing the silver halide, by heating and stirring together with or followed by the adding of binding agent, e.g., elatin, water and the usual emulsion ingredients, e'.g., chemical sensitizers, and

(8) Chemically ripening the emulsion.

According to a special embodiment of the invention, a part of the ammonium sulfate can be formed in situ, in the preparation of an ammoniacal emulsion during the coprecipitating of the gelatin and the silver halide, by adding, together with the concentrated solution of ammonium sulfate, sulfuric acid while the pH of the medium may not fall below pH 7. The sulfuric acid is preferably added as a mixture with the concentrated ammonium sulfate.

The concentration of the ammonium sulfate, dependent upon the pH of the precipitating medium, the type of gelatin, the size of the silver halide grains and the concentration of gelatin, varies preferably between 5 and 20% at the end of the precipitation. In the hardening stage the concentration of the ammonium sulfate amounts preferably to between 7 and 40%.

Distilled water, demineralized water and tap water preferably with a French degree of hardness not higher than 45 can be used as wash water.

The draining off of the supernatant liquid after cot2 precipitation of the gelatin and the silver halide is preferably carried out until the precipitate remaining amounts to about to /s part of the original volume.

The process according to the invention has, in comparison with the process of the U.S. patent specification 2,618,556, the advantage of avoiding a decrease of sensitivity of the ammoniacal emulsions by precipitating with acids below a pH of 7.

According to the above U.S. patent specification a large amount of sulfuric acid is needed with which ammonium sulfate is formed in situ by acidifying the ammoniacal emulsion, whereupon in a second step of the process occasionally more ammonium or sodium salt is added, in order to allow the silver halide dispersion to flocculate.

Further the U.S. patent specification has the disadvantage that only a small part of the gelatin precipitates and so some gelatin is lost when decanting. In this way, the solution keeps a certain viscosity and the settling of the precipitate is hindered. The supernatant, not completely clear solution still contains gelatin and can hold a part of the silver halide in dispersion which is lost when decantiug.

In the process according to the invention, however, practically the complete amount of gelatin which has been used for dispersing the silver halide is precipitated on the silver halide grains upon salting out.

After decanting the supernatant liquid from the gelatin silver halide coprecipitate grains, and in order to prevent these grains .from sticking together a second amount of ammonium sulfate is added and thoroughly stirred therewith, whereby a reversible hardening of the gelatin is obtained. During the washing with water of about C. the gelatin only swells moderately by this hardening so that there is no danger for peptization. The solubility of the precipitated and hardened gelatin, however, remains sufficiently high to enable redispersing by heating after washing.

The amount of gelatin used for the preparation according to the present invention of photographic lightsensitive silver halide emulsions can vary between 2 and g. per 100 g. of silver nitrate and depends on the size of the silver halide grains while the gelatin concentration of the silver halide dispersion before precipitating preferably does not exceed 3% by weight of the water present. A gelatin concentration from 0.5 to 2% is to be preferred.

In the precipitation of the above photographic emulsion a fiuocculent precipitate is immediately formed so that even with an extremely low gelatin concentration of the starting dispersion a powdery precipitate has settled at the bottom of the flask after 4 hr.

The formed precipitate consists of flocculent particles of gelatin containing silver halide crystals dispersed therein, so that the several crystals stay separated from each other.

The hardened gelatin particles of the precipitate are in a finely divided form so that they can be easily and rapidly washed with water, preferably at low temperature (:10" C.). After washing the precipitate is easily dispersed in water by heating and thereupon the desired amount of binding agent is added.

It is evident that according to the new process for the preparation of the emulsion the final amount of binding agent which is added to the light-sensitive salts can be chosen at pleasure so that emulsions can be obtained with a high amount of light-sensitive silver salt without increasing the content of gelatin and that a large amount of silver salts can be coated with less viscous emulsions onto relatively smaller surfaces. These emulsions with easily adjustable concentrations are especially suited for special coating techniques such as air-knife coating and extrusion.

By the physical ripening in the diluted gelatin solution the desired grain distribution of the silver halide is obtained. After removing the excess of water-soluble salts from the emulsion the light-sensitivity of the silver halide crystals can be increased by chemical sensitization.

The chemical sensitizers are mostly added after wash ing the emulsion. They may also be added during the precipitation of the silver halide or during the Ostwald ripening for instance when an active gelatin is used as dispersing agent for the silver halide.

It is known to sensitize chemically the emulsions either with sulfur sensitizers which probably form silver sulfide nuclei or with reducing agents which probably form silver nuclei or with gold compounds or other heavy metal compounds such as palladium or platinum.

In order to sensitize chemically the emulsion with sulfur sensitizers the emulsion can be ripened with active gelatin or sulfur compounds can be added to the silver halide emulsion such as allyl isothiocyanate, allyl thiourea, sodium thiosulfate or potassium selenocyanide.

The emulsions can also be sensitized with reducing agents such as tin salts or imino-aminomethane sulfinic acid compounds (German patent specification 1,020,864, filed April 28, 1956).

The highi-sensitive negative emulsions prepared according to the invention can especially be sensitized with gold (I) thiocyanate and sodium thiosulfate as described in French patent application 893,935, filed April 10, 1962.

In the preparation of emulsions according to theinvention ingredients may be added such as optical sensitizing dyestuifs, stabilizers, fog inhibiting agents, color couplers, hardening agents, wetting agents and matting agents.

The following examples illustrate the invention, wherein by parts are understood parts by weight.

Example 1 300 parts of silver nitrate are dissolved in 2900 parts of distilled water and then at 40 C., while vigorously stirring, added in 2 min. to a solution of 250 parts of potassium bromide, 45 parts of gelatin and 5 parts of potassium iodide in 1200 parts of water.

After the physical ripening which is carried out at 45 C. for about 30 min, 1400 parts of a 50% aqueous solution of ammonium sulfate are added while vigorously stirring. The silver bromide and gelatin immediately separate as a fine powder. When the precipitate has completely settled the supernatant clear solution is decanted till a volume of 1500 parts is left in the reaction flask. Then once again 200 parts of a 50% aqueous solution of ammonium sulfate are added and the mixture is stirred for 15 at 30 C. Then the precipitate is allowed to settle again and the supernatant solution is decanted till a volume of about 1200 parts. The mixture is washed for another four to five times; each time with 4000 parts of water and the supernatant solution is each time decanted till a volume of 1200 parts is left. After the last washing and decanting stages 600 parts of distilled water and 250 parts of gelatin are added.

Thereupon the mixture is stirred at 45 C. for 1 hr. and the emulsion is diluted with distilled water up to 3000 parts. Finally, the emulsion is chemically ripened at The emulsion is coated onto a cellulose ester support. A fog-free photographic material is obtained.

Example 2 250 parts of silver nitrate are dissolved in a mixture of 200 parts of concentrated ammonium hydroxide and 220 parts of water. This solution is then at once added to a solution of 570 parts of potassium'bromide, 10 parts of potassium iodide and 28 parts of gelatin in 2300 parts of water. Immediately thereafter a solution of 250 parts of silver nitrate in 220 parts of water and 200 parts of ammonium hydroxide are added in 5 min. The temperature of these solutions amounts to 52 C. The physical ripening takes place in 35 min. at 55 C. Then, while vigorously stirring 1300 parts of a 50% aqueous solution of ammonium sulfate is added. A precipitate of silver bromide and gelatin is formed which is allowed to settle for min. The supernatant mother-lye is decanted till a volume of 500 parts is left in the reaction flask. Then, while vigorously stirring 500 parts of a 50% aqueous solu tion of ammonium sulfate are added. The mixture is stirred for another 20 min. and then four times washed with 300 parts of water followed each time by decanting till a volume of 500 parts is left in the vessel.

After the last decantation 1200 parts of distilled water and 270 parts of gelatin are added and stirring is continued for 1 hr. at 45 C. T hereupon distilled water is added till a volume of 3000 parts is obtained. The chemical ripening of the emulsion takes place at 50 C.

The thus obtained emulsion is coated onto a cellulose triacetate support. A contrasty photographic material is obtained.

Example 3 100 parts of silver nitrate dissolved in 85 parts of water and 82 parts of concentrated ammonium hydroxide are added in 1 min. to a solution of 130 parts of potassium bromide, 5 parts of potassium iodide and 20 parts of gelatin in 880 parts of distilled water. The temperature of the solutions amounts to 50 C. After about 5 min. a solution of 150 parts of potassium bromide in 300 parts of water are added and then a solution of 150 parts of silver nitrate in 130 parts of distilled water and 125 parts of concentrated ammonium hydroxide are added in 4 min. The pH of the emulsion amounts at this moment to about 10. The emulsion is allowed to digest at 55 C. for 30 min. and then a mixture of 119 parts of concentrated sulfuric acid (98%) and 100 parts of ammonium sulfate in 150 parts of water is added. In this way the pH of the emulsion decreases till about 7.5. The pH must be kept above 7. By adding this mixture of sulfuric acid and ammonium sulfate the emulsion flocculates. The emulsion is allowed to settle for min. whereupon the supernatant mother lye is drained off till about part of the original volume. Thereupon once again a solution of 125 parts of ammonium sulfate in 180 parts of water is added which is followed by vigorously stirring for min. Then 1500 parts of water at 10 C. are added and after stirring for 10 min. the emulsion is allowed to settle for 15 min. The supernatant liquid is drained off till A; part of the original volume.

Washing is repeated for 4 to 6 times till the conductivity of the water is reached.

After the last washing step 130 parts of active gelatin and 1200 parts of water are added and the emulsion is dissolved by gradually increasing the temperature till 45 C. At this temperature the solution is stirred for 1 hr. The emulsion is then chemically ripened for 3 hr. at 45 C.

6 I claim: 1. A process for the manufacture of high sensitive neutral and ammoniacal silver halide emulsions comprising: (1) Preparing a silver halide dispersion by reacting a water-soluble halide with silver nitrate in a medium selected from the group consisting of neutral and ammoniacal aqueous media, said medium containing gelatin in an amount not in excess of about 2% and within the range of 2-25% of said silver nitrate, (2) Introducing into said dispersion a concentrated solution of ammonium sulfate in an amount sufiicient to co-precipitate said gelatin and silver halide but not suificient to decrease the pH of said medium below about pH 7,

(3) Draining oif the supernatant liquid,

(4) Admixing additional concentrated solution of ammonium sulfate to reversibly harden said coprecipitated gelatin,

(5) Washing and decanting said co-precipitant to remove soluble salts therefrom,

(6) Heating and stirring said co-precipitate to redisperse said silver halide, and

(7) Using the silver halide dispersion thus obtained to produce the ultimate light-sensitive silver halide emulsion.

2. In the process of claim 1 wherein said medium is an aqueous ammoniacal medium, the step of forming said concentrated solution of ammonium sulfate in situ by adding a less concentrated solution of ammonium sulfate containing sulfuric acid.

3. The process of claim 1 wherein said ammonium sulfate is introduced into said dispersion in step (2) in an amount sufficient to give a concentration of said ammonium sulfate of about 5-20%.

4. The process of claim 1 wherein said additional ammonium sulfate is admixed in step (4) in an amount sutficient to give a concentration thereof of about 740%.

References Cited in the file of this patent UNITED STATES PATENTS 2,126,516 Szasz Aug. 9, 1938 2,618,556 Hewitson et a1. Nov. 18, 1952 3,007,796 Steigmann NOV. 7, 1961 3,031,304 Oliver Apr. 24, 1962 3,067,035 Oberth et a1. Dec. 4, 1962 FOREIGN PATENTS 144,443 Australia Dec. 11, 1951 OTHER REFERENCES Wood: Journal of Photographic Science, vol. 9, 1961, pp. 151-156. 

1. A PROCESS FOR THE NAUFACTURE OF HIGH SENSITIVE NEUTRAL AND AMMONIACAL SILVER HALIDE EMULSIONS COMPRISING: (1) PREPARING A SILVER HALIDE DISPERSION BY REACTING A WATER-SOLUBLE HALDIE WITH SILVER NITRATE IN A MEDIUM SELECTED FROM THE GROUP CONSISTING OF NEUTRAL AND AMMONIACAL AQUEOUS MEDIA, SAID MEDIUM CONTAINING GELATING IN AN AMOUNT NOT IN EXCESS OF ABOUT 2% AND WITHIN THE RANGE OF 2-25% OF SAID SILVER NITRATE, (2) INTRODUCING INTO SAID DISPERSION A CONCENTRATED SOLUTION OF AMMONIUM SULFATE IN AN AMOUNT SUFFICIENT TO CO-PREICPITATE SAID GELATIN AND SILVER HALIDE BUT NOT SUFFICIENT TO DECREASE THE PH OF SAID MEDIUM BELOW ABOUT PH
 7. (3) DRAINING OFF THE SUPERNATANT LIQUID, (4) ADMIXING ADDITIONAL CONCENTRATED SOLUUTION OF AMMONIUM SULFATE TO REVERSIBLY HARDEN SAID COPREICIPITATED GELATIN, (5) WASHING AND DECANTING SAID CO-PRECIPITANT TO REMOVE SOLUBLE SALTS THEREFROM, (6) HEATING AND STIRRING SAID CO-PRECIPITATE TO REDISPERSE SAID SILVER HALIDE, AND (7) USING THE SILVER HALIDE DISPERSION THUS OBTAINED TO PRODUCE THE ULTIMATE LIGHT-SENSITIVE SILVER HALIDE EMULSION. 